1. Field of the Invention
The present invention relates to a method and an apparatus for detecting changes in an interval of time between an optical or electrical signal and an optical or electrical reference signal. In addition, the invention relates to a use of the method for synchronizing an optical or electrical signal with an optical or electrical reference signal.
2. Discussion of the Prior Art
It is important to synchronize optical or electrical signals in a highly precise manner in numerous time-critical fields of application, for example telecommunications, data transmission, surveying technology, navigation systems or in large research systems. In particular applications, it may be necessary to synchronize an optical or electrical signal with an optical or electrical reference signal in the range of femtoseconds, that is to say 10−15 s. For such precise synchronization, it is necessary to detect changes in the interval of time between two signals in a highly precise manner in order to then be able to stabilize the interval of time between two signals.
Since, in one femtosecond, light covers a path length of only approximately 0.3 μm, it immediately becomes clear that even minimal changes in length, for example as a result of thermal expansion of optical components, may result in changes in the interval of time between an optical signal and an optical reference signal. This concerns, in particular, the transmission of light signals in a long glass fibre optical waveguide. In order to be able to correct any changes in the length of the transmission path, the change in the interval of time between an optical signal and an optical reference signal must be detected to the femtosecond.
In particular, in order to operate tree electron lasers in the UV or X-ray range, for example the free electron laser in Hamburg (FLASH) and the European free electron laser (XFEL), it is necessary to synchronize various components in the accelerator to the femtosecond. In the case of the XFEL, the components to be synchronized are at a distance of up to 3.5 km from one another, with the result that coaxial distribution systems reach their limits.
A reference pulse laser is typically used to transmit a common optical reference signal to all components to be synchronized. The reference pulse laser itself is usually synchronized with an electrical original reference signal which is predefined by a microwave oscillator, for example. The components to be synchronized with the reference pulse laser beam use either optical or electrical signals which have to be synchronized with the optical reference signal from the reference pulse laser. Such a component in an accelerator could be, for example, an arrival time monitor which is used to determine the arrival time of electron pulses. For this purpose, the arrival time monitor requires an optical or electrical signal which is synchronized, for example, with the signals from other arrival time monitors at other locations in the accelerator. Therefore, all arrival time monitors use the common optical reference signal from the reference pulse laser. However, the problem in this case is that each branch of the reference signal to a component is exposed to different external conditions, for example temperature influences, and the path lengths of the reference signal to the individual components are therefore subjected to fluctuations which are not correlated with one another and interfere with the synchronization of the signals.
It is known from the prior art to use a non-linear crystal to correlate two optical pulse signals which overlap and to use a steep edge of the correlation for highly precise synchronization. However, the disadvantage of the known methods is that the correlation is dependent on the polarization of the signals. In addition, the method is highly dependent on the pulse lengths which, for the rest, have to overlap in terms of time.